1. Technical Field
The invention relates to devices for isolating a narrow portion of a spectrum. More particularly, the invention relates to an oscillating mechanism driven monochromator.
2. Description of the Prior Art
A monochromator is an optical instrument that is designed to separate polychromatic white light (i.e. light consisting of more than one color or wavelength) into monochromatic light (i.e. light of a single color). State of the art monochromators typically use a Czerny-Turner optical system (see FIG. 1). Light enters an entrance slit 11 and is collected by the collimating mirror 13. Collimated light strikes a grating 15 and is dispersed into individual wavelengths (i.e. colors). Each wavelength leaves the grating at a different angle and is re-imaged at the exit slit 17 by a focusing mirror 19. Because each wavelength images at a different horizontal position, only the wavelength imaged at the exit slit opening is allowed to exit the monochromator. Varying the width of the entrance and exit slits allows more (or fewer) wavelengths of light to exit the system.
Rotating the diffraction grating scans wavelengths across the exit slit opening. The monochromatic light produced by a monochromator can be used to illuminate a sample, or it can be scanned across a detector and measured for intensity at individual wavelengths. Conventional monochromators use an oscillating grating to perform such spectral separation. The grating is typically oscillated by a drive mechanism.
There are presently three types of drive mechanisms commonly used in monochromators.
FIG. 2 shows a conventional sine drive scanning system with precision drive screw 21, drive block 23, and sine bar 25. In such drive mechanism, a motor 27 turns a drive screw which moves a drive block which, in turn, pushes the sine bar to rotate the grating.
FIG. 3 shows a direct digital scanning system with worm/worm gear arrangement. In this drive mechanism, a stepping motor 37 turns a worm gear mechanism 31 which rotates a grating turret 33 a full 360xc2x0, thereby allowing for wavelength scanning and grating changes.
FIG. 4 shows a direct digital scanning system that rotates the grating about its face (on-axis). In this drive mechanism, a motor 47 turns a worm gear 41 to change wavelengths. A second motor or complex mechanical arrangement 45 changes the gratings.
Various problems stem from the use of such mechanisms. One such problem is the issue of position repeatability of the grating""s zero order position. Another problem with such mechanisms is the dependency of the torque profile on direction. The sine bar mechanism also requires that the drive motor be reversible to produce the oscillation of the grating, while the direct digital scanning system is unnecessarily complex.
Rather than using the above mechanisms to produce the oscillation of the grating, some monochromators incorporate a direct coupling of a reversing motor to the grating. This design requires that the motor be reversible and have high torque characteristics.
Such state of the art monochromators lack stability and accuracy and require high torque drive motors. While this is not a problem for some applications, it a serious limitation for applications where such imperfections in the monochromator drive mechanism add a significant amount of noise to a signal developed with the monochromator, for example where the monochromator is a signal source in an instrument for noninvasive measurement of blood analytes.
It would be advantageous to provide a monochromator that overcomes the deficiencies known in state of the art devices, especially with regard to the use of such monochromators in precision applications.
The invention provides a mechanism for oscillating the spectral grating of a monochromator. The mechanism couples the spectral grating of the monochromator to an oscillating spatial linkage mechanism which accepts a rotational input and converts it into an oscillatory motion. A monochromator according to the invention comprises an oscillating grating that is oscillated by such spatial linkage mechanism drive.